ORLANDO, Florida (CNN) -- Something in the lakes around Orlando, Florida, has claimed the lives of three boys this summer.

Will Sellars' family says he died after being exposed to a deadly amoeba on a Florida lake.

"This thing is just there. It's lurking like some deadly thing in the water which can take our children's lives and we all have to be aware," said Orange County Health Department Director Dr. Kevin Sherin.

The "thing" isn't a fish or alligator. It is so small it cannot be seen with the naked eye. The killer that lives in the hot, fresh water is a single cell amoeba that once exposed to the human brain through the nasal passages is almost always fatal.

At first people exposed to the amoeba, naegleria fowleri, suffer from flu-like symptoms. Very quickly, in from one to 14 days, the symptoms worsen, Sherin said. "There's a downhill course. Folks lapse into a coma; there are abnormal movements of the eyes and a terrible cascade of events leading to the actual death of parts of the brain."

Sherin said exposure to the amoeba can be detected by an MRI and it can be treated with antibiotics if caught early enough, but Sherin said he believes medical personnel are not in the habit of looking for the disease.

That is because the amoeba is very rare. The Centers for Disease Control in Atlanta, Georgia, has documented 24 cases in the United States since 1989.

Health officials do not know what caused three cases in Orlando in one summer. Theories range from warmer temperatures to a drought that has lowered lake levels. Sherin said officials considered closing access to the lakes, but concluded they did not have the authority. Even if public lakes had been closed, private lakes would have remained open.

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So, at 15 parks and lakes around the city, warnings about the amoeba have been posted. The signs urge bathers to wear nose clips or stay out of water warmer than 80 degrees Fahrenheit, which can be a breeding ground for the amoeba.

The warnings provide little solace for Steve Sellars.

Health investigators said they believe Sellars' 11-year old son, Will, was exposed to the amoeba during an August weekend spent learning to wakeboard on Orlando's Lake Jessamine.

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A month later, a 10-year-old boy died from exposure to the amoeba. Investigators have not determined where he was exposed. The death of a 14 year-old boy in June in the Orlando area also is being blamed on the amoeba.

As he investigates the deaths of the three boys from the amoeba, Sherin is concerned these type of deaths may be underreported. Health departments in Florida are not required to report amoeba infections to the state. The illness is so rare, he said, it may be commonly misdiagnosed in the United States and internationally.

He said anyone who exhibits flu-like symptoms who has been in a lake recently should see a doctor immediately.

Speaking in Will's old bedroom, which Steve Sellars has decorated with photographs of his son, Sellars said he hopes he can help get the word out. He does not want anyone to lose a family member as quickly and mysteriously as he did.

"It's the worst thing we ever had to go through and I hate to see any other parent go through this and another child lose his life," Sellars said.E-mail to a friend